The present invention relates to an apparatus for manufacturing tubing from sheet stock using a series of rolls. In particular, the invention relates to an improved tube forming machine and method which utilizes new roll shapes and bending techniques which provide a number of advantages over prior art machines and methods.
Steel tubes have for many years been produced by forming an initially flat sheet or strip into a round shape using cage rolls, cluster rolls and fin-pass rolls, and eventually welding the edges of the sheet together to form a seam. Conventional equipment utilizing such rolls for the formation of steel tubing from strips can be seen in U.S. Pat. Nos. 5,673,579 and 5,784,911.
Because a large component in the cost of producing steel tubing from sheet material is the cost of the sheet material itself, producers of steel tubing are often forced by competition to use the least expensive sheet steel available. However, inexpensive sheet stock often has more variability in the hardness, thickness and other important properties of the sheet as compared to more expensive sheet stock. When inexpensive steel sheet is used with traditional tube forming machines and techniques, a number of problems arise. Those problems include twisting of the sheet as it passes through the various rolling stands, difficulty in controlling the position of the sheet, and difficulty in feeding the sheet at the start of a continuous tube forming operation. Conventional tube forming machines require rolls to be changed frequently in order to form tubing having different sizes and wall thicknesses. It is therefore desirable to provide a tube forming machine which has improved ability to handle inexpensive sheet steel and which has increased capacity to make tubing from different forming rolls.
Important objectives in the design of tube forming equipment include ease of initial threading of the strip into and through the machine, consistent positioning of the sheet both at the forming stands and at the point in the process where the edges of the sheet are welded to form a seam, efficient handling of the strip without damaging either the edges or the surfaces of the sheet, and ability for the machine to handle a wide range of tubing sizes and wall thicknesses without changing the forming rolls.
The present invention utilizes three point bending techniques at various stages in the tube forming operation. One of the three point bending techniques of the present invention involves the use of a V-shaped roll and an opposing narrow roll, with the extent of curvature obtained depending on the relative position, i.e., the proximity, of the two roll. If the narrow roll is brought closer to the V-shaped roll with which it cooperates, a smaller diameter is obtained. Conversely, if the gap between the narrow roll and the V-shaped roll is increased, a larger diameter results. The present invention also utilizes a V-shaped bottom and top roll at an initial or pinch roll stand. The flat surfaces of opposing V-shaped rolls at the first stand in the machine results in improved gripping of the sheet for purposes of driving the sheet through subsequent stands. The resulting V-shaped profile of the sheet after it leaves the initial pinch roll stand is a strong shape for purposes of driving the sheet as it is threaded through the remaining non-driven stands. The initial forming stand is equipped with a duplex regulating system in which hydraulic pressure is used to pinch the sheet between the two V-shaped rolls. Each side of the top roll of the initial station may be independently controlled for purposes of adjusting pressures applied to each side of a sheet being processed to compensate for variability of thickness of the sheet material.
The tube forming machine described below also includes the use of a brimmed roll in which a circumferential slot is formed between two angled surfaces. A pair of brimmed rolls are used to engage the edges of a sheet, and the two brimmed rolls cooperate with a concave bottom roll to form the sheet into a smoothly rounded cross-section.
More detailed descriptions of the inventions disclosed herein are set forth below and will be better understood upon a reading of the following specification read in conjunction with the accompanying drawings wherein: